This invention relates to an image display tube (or a picture tube) which provides a high-quality display image and which is composed of in-tube parts such as a shadow mask, a frame and an inner shield comprising a permanent-elasticity alloy (hereinafter "Elinvar alloy") excellent in moldability and thermal properties.
An image display tube generally has a structure shown in FIG. 1. In this structure, a neck portion 2 constituting one end portion of a glass outer surrounder 1 is provided with, for example, an in-line electron gun 3, and a face portion 4 constituting the other end portion of the glass outer surrounder 1 opposite to the electron gun 3 is provided on the inner surface thereof with a fluorescent surface 5 on which red, blue and green fluorescent substances are sectionally arranged. A shadow mask 6 having a number of beam openings is disposed adjacent to and opposite to the fluorescent surface 5. This shadow mask 6 is fixed to a frame 7 with the interposition of an engaging tool 8, and this frame 7 is provided with an inner shield 9 so as to block the influence of earth magnetism.
In the image display tube thus constituted, electron beams 11 irradiated from the electron gun 3 are deflected by a deflecting device 10 disposed around a root section of the neck portion 2, pass through the openings in the shadow mask 6, and hit against the fluorescent surface 5 in order to generate fluorescents, thereby forming an image thereon.
In this case, the shadow mask 6, the frame 7 and the inner shield 9 have good etching properties and moldability, and are made from a material such as rimmed steel or aluninum killed steel, on a surface of which there can easily be formed oxide membrane capable of reducing the reflection of the electron beams. However, in order to satisfy requirements for a variety of recent communication media, higher quality of the image receiving tubes is required, in other words, it is required that the displayed image is easy to watch and is extremely fine. As a result, it is getting inconvenient to use the shadow mask 6, the frame 7 and the inner shield 9 comprising the above-mentioned rimmed steel and aluminum killed steel.
That is, when the image display tube is operated, temperature of the respective parts will rise up to a level of 30.degree. to 100.degree. C., and a strain will occur, for example, on the molded shadow mask owing to the thermal expansion of these parts and finally the so-called doming will appear thereon. As a result, relative deviation of the shadow mask from the fluorescent surface will take place, so that color deviation called "purity drift" (PD) will appear there. Particularly in the case of the high-quality image display tube, since the diameter and pitch of the openings in the shadow mask are very small, the aforesaid deviation will be relatively great, which fact will cause the in-tube parts comprising the above-mentioned rimmed steel and aluminum killed steel to become impracticable. In particular, the high-curvature type image receiving tube in which the reduction in the strain of images and the reflection of external lights is intended will bring about this problem remarkably.
Accordingly, it has heretofore been proposed, for example, in Japanese Patent Publication No. 25446/1967 and Japanese Unexamined Patent Publication Nos. 58977/1975 and 68650/1975 to use an Ni-Fe alloy having a small thermal expansion coefficient, for example, an invar (36Ni-Fe) as a formation material for this type of in-tube parts. Even in such Fe-Ni alloy, however, its temperature has risen owing to the bombardment of electrons and a color deviation has consequently occurred, which fact has elucidated that such a proposed technique is insufficient to heighten the quality of the image display tubes.